Supersonic virtual impactor

ABSTRACT

A supersonic gas flow is employed with a virtual impactor to separate fine particles completely from the gas. The carrying gas and fine particles are accelerated to supersonic speeds and then impacted against a virtual impactor. When the supersonic stream strikes the virtual impactor, a shock wave forms in the gas stream near the impactor surface. The carrying gas turns sharply away while the particles in the gas stream, carried by their inertia, continue in their original direction and pass into the virtual impactor. On the downstream side of the virtual impactor surface, a non-contaminating inert gas maintains a pressure equal to or greater than the pressure of the carrying gas between the virtual impactor surface and the shock wave. By using a supersonic flow to carry the particles, the carrying gas can be effectively completely separated from the particles and the minimum size of particles that can be separated from the carrying gas can be reduced from those achievable by conventional prior art subsonic flow virtual impactors.

This invention was made with Government support under ContractN0014-90-C-0177 awarded by the Department of Navy. The Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and an apparatus for separating fineparticles from a carrying gas by accelerating the carrying gas to asupersonic speed and impacting it against a virtual impactor so as toform a shock wave at the surface of said virtual impactor and collectingthe fine particles that pass through the surface of the virtual impactorin the interior chamber of the virtual

2. Description of the Prior Art

The separation and collection of fine particles from gases is of intenseinterest in material sciences where fine particles may have unique andvaluable properties. Often such particles are produced along withbyproduct vapors in high temperature environments. It is frequently ofimportance to preserve the purity of the particles by separating themfrom the byproduct vapors which may condense on them if the particlesare simply filtered.

The separation and collection of fine particles from gases is alsoimportant in preventing such particles from entering the atmosphere asan unintended consequence of manufacturing or power generationprocesses, as in the manufacture of cements and the fly ash producedfrom coal-fired electrical generators. Large investments are made infiltration systems and/or high voltage electrical devices to separateand collect the fine particles that can cause pollution.

The separation and collection of fine particles from gases is alsoimportant in research on atmospheric aerosols and particulates and inpreparing powders comprising particles of uniform size for suchapplications as advanced materials processing.

Prior art devices known as virtual impactors are often used to classifyfine particles suspended in carrying gas according to their sizes. Inthese devices, a subsonic gas stream containing particles is caused toimpinge upon a surface containing an aperture. The flow of the impinginggas/particle stream into the aperture is controlled in such a way thatonly a small fraction of the impinging flow passes into the aperture.The majority of the gas in the stream and the small particles whichfollow the gas flow are forced to turn away from the aperture. Largerparticles, with greater momentum, cannot negotiate the turn and followthe smaller (minority) flow into the aperture. The minority flow and thelarger particles are carried through the aperture to a collection devicefor the particles, such as a filter. The majority flow and the smallerparticles are passed into a separate collection device. Very accurateand balanced control of the majority and minority flows is used todetermine the size cutoff between particles passing through the aperturewith the minority flow and those which follow the majority flow.Successive stages of virtual impactors may be used to further classifythe smaller particles in the majority flow. The smaller the particles,the greater their tendency to follow the gas flow, and thus the moredifficult they are to classify, requiring ever more accurate flowcontrol and geometric tolerances.

Another kind of prior art virtual impactor, called a counterflow virtualimpactor, is used to attain closer control of the size cutoff betweenparticles collected from the minority flow and those retained in themajority flow. In the counterflow virtual impactor, a particle laden gasflow is caused to impinge upon a surface containing an aperture asdescribed. In this case, the apertured surface is formed by a solidplate joining together two concentric tubes. The outer tube has a solidwall and the inner tube has a porous wall for a short distance near itsend joining the solid plate. The inner tube forms the aperture and thesolid plate joining it to the outer tube forms the solid surface of thevirtual impactor. Gas is supplied to the annular space between the tubesand passes through the porous part of the inner tube into the spacebehind the aperture. A suction is applied to the end of the inner tubeaway from the aperture. Part of the gas added through the porous wall isdrawn into the suction and part flows toward the aperture. Because ofthis difference in flow direction, there is a plane in the porous tubeat which the added gas flow has zero velocity along the axis of theporous tube. This plane lies within the porous tube behind the apertureand solid plate which form the surface of the virtual impactor.Particles impacting the apertured surface either penetrate the apertureor are turned aside depending on their size and velocity. Thecounterflow of gas from the porous tube provides an additional selectionmethod by forcing those particles which are collected to travel somedistance within the porous tube in order to pass the plane of zero gasvelocity. Those which do pass this plane are entrained in the gas flowdrawn by the suction and may be collected by a device such as a filter.Those particles which do not reach the zero velocity place are expelledto rejoin the majority flow turned aside at the virtual impactorsurface. The gas flow through the porous tube may be varied in order tomove the position of the plane of zero velocity, thereby selecting thepenetration distance of the particles which are collected. Thepenetration distance depends upon the particle size, and thus selectingthe penetration distance is effective in selecting the size cutoff ofthe particles collected--for a given penetration distance, particleslarger than a certain size will be collected while smaller particleswill be rejected.

There is some limited discussion in the prior art patent literature withregard to supersonic flows in the context of virtual impactors, however,the purpose and function of those flows is entirely different from thesubject matter of the present invention. For example, U.S. Pat. No.4,806,150 entitled DEVICE AND TECHNIQUE FOR IN-PROCESS SAMPLING ANDANALYSIS OF MOLTEN METALS AND OTHER LIQUIDS PRESENTING HARSH SAMPLINGCONDITIONS by Joseph L. Alvarez and Lloyd D. Watson discloses the use ofsupersonic flows in a device that includes a virtual impactor. Thepurpose of the supersonic flow is to break up injected liquid and toprovide more effective cooling of the particles in the flow than isfound in subsonic atomizers. There is no teaching or suggestion ofimpacting the supersonic flow against a virtual impactor so as toprovide for particle separation or sizing.

U.S. Pat. No. 5,021,221 discusses a solid plate impact separatorutilizing the impingement of a particle laden supersonic stream onto asolid plate for the separation of fine liquid particles from gases Thatdevice is incapable of separating solid particulates from gases becauseit requires the material being collected to "stick" when it strikes thesurface, so that it, for example, forms a thin liquid film on thesurface of the solid impactor plate to aid in sticking of newly arrivedliquid particles. In addition, the disclosure in the cited patent doesnot teach or suggest collection or separation of solid particles. Thepresent invention will separate either solid or liquid particles fromgases, but the separation and collection of solid particles is thepreferred use.

Similarly, Russian Patent SU 693 162 discusses an impactor that usessupersonic flows to collect small particles on a side wall, butotherwise appears to be irrelevant to the subject matter of the presentinvention.

The following patent references appear to be of lesser relevance: U.S.Pat. Nos. 2,793,282; 3,077,307; 3,430,289; 3,602,595; 3,659,944;4,301,002; 4,670,135 and 4,767,524.

A useful discussion of the physical principles relevant to the presentinvention is set forth in the following reference texts: THE DYNAMICSAND THERMODYNAMICS OF COMPRESSIBLE FLUID FLOW, VOL. I AND II, by A. H.Shapiro (Ronald Press, N.Y. 1954) and HYPERSONIC FLOW THEORY by W. D.Hayes and R. F. Probstein (Academic Press, N.Y. 1966).

It was in the context of the foregoing prior art that the presentinvention arose.

SUMMARY OF THE INVENTION

Briefly described, the invention comprises a supersonic virtual impactorfor separating fine particles from a carrying gas stream. The deviceseparates fine particles from carrying gas by forming a directedsupersonic stream of the carrying gas containing the particles andimpacting the stream against a wall or shaped body which includes anaperture. When the stream strikes the wall or body, a shock wave formsin the gas stream near the surface which delineates the large,instantaneous decrease in gas velocity from supersonic upstream of theshock wave to subsonic downstream of the shock wave. Downstream of theshock wave, the carrying gas turns aside; the particles, carried bytheir inertia, continue in their original direction, passing through theaperture in the wall or body. On the far side of the aperture, anon-contaminating inert gas maintains a pressure equal to or greaterthan the pressure of the carrying gas between the wall and the shockwave. Particles thus pass from the carrying gas into the inert gasbehind the wall, while the carrying gas turns aside. The particlespenetrate for a certain distance into the inert gas until drag forcesslow their velocity to the gravitational settling velocity. At thisdistance, the particles are transported by flowing the inert gas to acollection means, e.g., a filter.

The major object of the present invention is to provide an improvedmeans for separating and collecting fine particles from carrying gases.The carrying gases may be at high temperatures and/or may becondensible. The minimum size of particle that can be separated from thecarrying gas is smaller than that achievable with conventional prior artsubsonic flow virtual impactors. In addition, in the prior art, it iscommon for the carrying gas, if it is a condensible vapor, tocontaminate the particles should it be deposited with them on a filterinside of the virtual impactor. According to the present invention,however, virtually none of the carrying gas accompanies the particlesseparated from the supersonic gas flow stream.

These and other features of the present invention will be understood byreferring to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a detailed cross-sectional view of the region close to thesurface of the supersonic virtual impactor illustrating the separationof particles from the carrying gas; and,

FIG. 2 is a cross-sectional view of the method and apparatus of thepresent invention for separating and collecting particles from acarrying gas.

FIG. 3 illustrates the impactor assembly with concave plate 532.

DETAILED DESCRIPTION OF THE INVENTION

During the course of this description, like numbers will be used toidentify like elements according to the Figures that illustrate theinvention.

The novel method of the present invention is to form a supersonic streamof the particle laden carrying gas and to impinge that stream upon avirtual impactor. Doing this improves the performance of the virtualimpactor as a particle size classifier, and it has the consequence ofmaking it a device which separates the particles essentially completelyfrom the carrying gas. It has the further consequence that for particlesin the 0.1-10.0 μm size range, essentially all of the particlesimpinging on the virtual impactor may be collected free of the carryinggas in a single pass. No subsonic virtual impactor or subsoniccounterflow virtual impactor can achieve this essentially completeseparation for such a large range of particle sizes in a single step.Thus, the supersonic virtual impactor is useful as a method andapparatus to separate particles from carrying gas. Additionally, thesupersonic virtual impactor is useful as a classifier for smallparticles since the size cutoff of particles may be as small as 0.1 μm.Thus, such small particles may be separated from even smaller particleswhich are carried away, in the language of the prior art, with themajority flow. This size cutoff is smaller than can be achieved withsubsonic prior art virtual impactors.

According to the preferred embodiment of the present invention, by usinga supersonic flow to carry the particles, the particles may beessentially completely separated from the carrying gas and the minimumsize of particle that can be separated from the carrying gas can bereduced from that achievable with conventional prior art subsonic flowvirtual impactors. There are two reasons for this: 1) the momentum ofthe particles is larger (i.e. particles of the same size have a highervelocity in the supersonic flow than they would in a subsonic flow) and2) the travel times and distances over which aerodynamic drag forceswhich could cause the particle trajectory to miss the collectingaperture operate are smaller in the supersonic flow.

This latter point is illustrated in FIG. 1 which schematically shows adetailed cross section of the region very close to the surface of thesupersonic virtual impactor. In FIG. 1, a cylindrical particle ladensupersonic gas stream 31 of diameter, d, is shown impinging of thesurface of a virtual impactor, comprising a solid plate 431 and anaperture 432 from which is issuing a counterflow of inert gas 437. Theimpingement causes a shock wave 32 to be formed in the supersonic stream31 at a distance D_(sw) from the surface of the virtual impactor. Thedistance D_(sw) is approximately equal to d/4. For illustrativepurposes, two sizes of particles are shown in the supersonic stream 31,the larger 50 being envisioned to be about 3 μm in diameter and thesmaller 51 being less than 0.1 μm in diameter. In the region upstream ofthe shock Wave 32 (above the shock wave 32 in FIG. 1) aerodynamic dragforces cause the particles to move in a direction perpendicular to thevirtual impactor surface. Downstream of the shock wave 32, the carryinggas 33 is forced to turn sharply away from the obstacle presented to itby the virtual impactor surface, and aerodynamic drag forces act to pullthe particles with the carrying gas 33 parallel to the impactor surfaceso as to cause them to veer away from the aperture 432. The largeparticles 50 have sufficient mass and velocity to overcome these dragforces and to penetrate into the aperture 432 separated from thecarrying gas 33 for subsequent collection. The small particles 51 aredragged away from the aperture with the flow of carrying gas 33.

As FIG. illustrates, drag forces that negatively influence particletrajectories (i.e. cause them to miss the collecting aperture) operateonly in the region downstream of the shock in the supersonic flow case.Upstream of the shock wave, drag forces direct the particles toward theimpactor aperture. The shock wave is the boundary of a sharpdiscontinuity in the carrying gas flow speed and direction occurringonly in supersonic flow. By comparison, in a conventional prior artsubsonic flow virtual impactor, gas flow follows smoothly curvingstreamlines from the location at which the subsonic stream is formed allthe way to the impactor surface. The curving streamlines result from thefact that the presence of the impactor surface in the flow iscommunicated upstream against the flow at the velocity of sound in thecarrying gas. Thus drag forces which act to cause the particles to missthe aperture operate over the full flow distance of the subsonic streamwhereas the analogous drag forces act only over the small distancebetween the shock wave 32 and the impactor surface 431 in the supersonicflow case.

FIG. 2 is a cross sectional diagram illustrating the present inventioncomprising an upstream section 100 into which particle laden ga isadmitted through a tube 101. The upstream section is bounded by a nozzlesection 200 through which the particle laden gas flows into thedownstream section 300. The supersonic stream 10 formed by the nozzle200 impinges on a virtual impactor assembly 400 located within thedownstream section 300. Particles are separated from the carrying gasand enter a filter chamber 500 where they are collected on a filter 501.The separated carrying gas exits the downstream section through a tube301 (or, if it is condensible, it may be condensed upon the walls of thedownstream section 300).

The formation of a supersonic stream 10 of the particle laden carryinggas is effected by passing the gas through a nozzle 200 which may beshaped according to the known arts for such structures, e.g., aconverging cylindrical tube, a converging-diverging cylindrical tube(known as a de Laval nozzle) or the equivalent structure in which theopenings are rectangular slits rather than circular. The convergentsection of the nozzle 201 concentrates the particles toward the centerof the flow through the nozzle. The carrying gas pressure in theupstream section 100 where it enters the nozzle 200 is kept at least twotimes larger than the gas pressure at the exit of the nozzle 204 whereit enters the downstream section 300 either by compressing the upstreamgas or applying a vacuum to the downstream gas. The ratio of pressuresbetween the upstream section 100 and downstream section 300 determinesthe velocity of the resulting stream, which will be supersonic wheneverthe pressure ratio exceeds about 2. When the carrying gas and particlesexit the portion of the nozzle which has the smallest cross-section (the"throat" 202), they begin to experience the reduced pressure of thedownstream section and the carrying gas expands and accelerates. Theinertia of the particles tends to keep them in the core 11 of theexpanding gas stream 10 thus resulting in a focusing effect in whichparticles are focused into a core 11 due to the fact that outward radialacceleration of the particles is smaller than that of the gases. Theability of the particle trajectories to follow or separate from gas jetstreamlines determines the focusing effect, and consequently largerparticle sizes will be focused more than smaller particles.

The downstream section of the nozzle 203 may be a diverging section asin a de Laval nozzle or the gas may be allowed to expand freely. A deLaval nozzle may be used which is designed so that the divergence of thenozzle exactly matches the gas expansion from the pressure at the throat202 to the pressure at the downstream section 300. When this is done,the supersonic stream 10 formed by such a nozzle maintains its shape anddoes not expand further beyond the cross-sectional area which it has atthe nozzle exit 204. The supersonic stream thus formed will contain acore 11 of particle-laden carrying gas surrounded by a flow of theremaining carrying gas. The total cross-sectional area of the stream 10will be approximately that of the exit of the shaped nozzle 204; thecross-sectional area of the particle-laden core 11 will be approximatelythat of the throat 202. The cross sectional shapes of the stream 10 andcore 11 will be those of the nozzle exit 204 and throat 202,respectively.

At some distance, denoted x, from the nozzle exit 204, the virtualimpactor assembly 400 is placed such that the supersonic stream impingesupon it. The distance x may be chosen as suits convenience ofconstruction for an ideal expansion using a de Laval nozzle and this isthe preferred mode of operation. For operating conditions deviating fromideal, x is chosen preferably to be less than the distance at which ashock wave would form if the downstream section 203 of the nozzle wereeliminated, i.e., in the case of free expansion. For such a situation,the distance x is chosen according to the formula x<0.67·d_(t) ·(P₀/P_(d))^(1/2), where d_(t) is the diameter of the nozzle throat 202, P₀is the pressure in the upstream section 100, and P_(d) is the pressurein the downstream section 300. As an example, for a pressure ratio, P₀P_(d) =10 across a nozzle with d_(t) =1 cm, x would be chosen preferablyless than about 2.1 cm for operating conditions deviating from ideal.

The size of the virtual impactor aperture 402 is preferably that of thecore 11 of the supersonic stream 10 at the selected distance. In thiscontext, "size" means cross sectional area and shape. For the ideal deLaval nozzle, this size is approximately equal to that of the nozzlethroat 202. The aperture may be chosen larger, up to a sizeapproximately equal to that of the supersonic stream at the selecteddistance. The cross-sectional area of the supersonic stream may beestimated as a function of distance from the nozzle by methods describedin the Reference texts.

The front face 401, which contains the aperture 402, of the virtualimpactor assembly 400 may be a flat plate or may be shaped in a mannerknown in the art (described in the Reference text by Hayes andProbstein) to achieve a stable shape and standoff distance, D_(sw), forthe shock wave 12. FIG. 3 illustrates the virtual impactor assembly withconcave plate 532. The objective in shaping the front face 401 is tominimize the standoff distance, D_(sw), which as indicated, is thedistance over which drag forces act to cause particles to miss theaperture. Thus a convex curvature or conical shape of the front facepermits a smaller D_(dw) than does a flat face, further reducing thedistance over which drag forces can negatively affect particletrajectories.

Inert gas, for example argon or nitrogen or another gas which isunreactive with the supersonic stream gas or the particles, isintroduced via conduit 404 into the impactor plenum 403 which is influid communication with the impactor chamber 408 to maintain thepressure in the impactor chamber slightly greater than the pressurebehind the shock wave 12, the recovery pressure, which may be calculatedaccording to methods known in the art. In general, the recovery pressureis approximately equal to the pressure in the upstream section 100.

Subsequent to their separation, the particles are collected from theinert gas on a filter 501. The removal of the carrying gas from theparticles by the supersonic virtual impact collector drastically reducesthe filter area required compared to separating the carrying gas fromthe particles directly.

If the material of construction of the nozzle 200 and impactor plate 401are refractory and able to withstand high temperatures withoutdistortion or reaction with the carrying gas, then the supersonicvirtual impactor 400 can separate particles from very high temperaturegases. Such high temperature gases might be generated as a part of achemical synthesis, a combustion process, or as the result of anelectric discharge. The particles to be separated may be either thedesired product or an undesirable byproduct which must be removed beforethe gas cools, for example, when the carrying gas is condensible. It isa major advantage of the method and apparatus of this invention thatvery high temperature condensible carrying gases may be separated fromfine particles without condensation of the carrying gas onto theparticles and without the use of large are high temperature filters.

The following examples are illustrative of the conditions for the methodof the invention and it is understood that the scope of the invention isnot limited by them.

EXAMPLE 1

In the apparatus of FIG. 2 a carrying gas, air, at a pressure of about500 kPa (5 atmospheres) within which are the 1 μm diameter particles itis desired to separate and collect is brought through a conduit 101 tothe upstream section 100. The carrying gas and particles exit through asteel de Laval nozzle 200 with throat diameter of 0.1 cm into thedownstream section 300 open to the air through a conduit 301 where thepressure is about 100 kPa (1 atmosphere) forming a supersonic stream 10of velocity 380 m/s and diameter 0.22 cm. At a distance of 10 cm fromthe nozzle exit 204, a circular 2 cm diameter impactor plate 401 isplaced perpendicular to the stream formed by the nozzle. In the centerof the plate is a conical circular aperture 402 0.3 cm in diameter. Thecenter of the impactor plate 401 is exactly aligned axially with thecenter of the nozzle 200. The aperture 402 forms the entrance of acylindrical porous steel tube 405 forming the impactor chamber 408 intowhich air slowly bleeds from the impactor plenum 403 through the poroustube 405 in order to exert a pressure within the impactor chamber 408slightly greater than that exerted by the impacting supersonic stream10, 500 kPa, and so there is a small flow of air out of the aperture402, opposing the supersonic stream flow. At the opposite end of theporous tube, a filter chamber 500 containing a filter 501 on which theparticles are collected is disposed in communication with the impactorchamber 408. A small suction is applied via a tube 502 to the chamber500 through the filter 501 so that some of the air which enters theporous tube 405 is drawn through the filter 501 (and some of it exitsthe aperture 402).

A shock wave 12 having a shape similar to that of the impactor plate isformed in the supersonic stream 20 about 0.05 cm above the surface ofthe impactor plate 401. The air in the supersonic stream 20 is turnedaside at the impactor plate 401 flows into the downstream section 300where it exits to ambient via a conduit 301 while the particles continuein their original direction at a velocity of 380 m/s, penetrating intothe impactor chamber 408 where they are slowed by the air flow therein.At a distance of 1.3 cm from the shock wave 12, the particles haveslowed to their gravitational settling velocity and the air flow in theimpactor chamber 408 has been adjusted so that the direction of flow istoward the chamber 500 containing the filter 501. The particles areentrained in the flow and are conveyed thence to be collected on thefilter 501.

EXAMPLE 2

It is desired to collect valuable 3 μm diameter, pure boron powders froma process stream carrying gas comprising sodium chloride vapor in whichthe particles are suspended without contaminating the particles bycondensing sodium chloride o them using the apparatus of FIG. 2. Thecarrying gas with suspended fine particles is brought via conduit 101 tothe upstream section 100 connected to a graphite nozzle 200 at atemperature of 1900K and a pressure of 25 kPa. The entrance section ofthe nozzle 201 tapers in at a half angle, A1, of approximately 20° tothe throat which has a diameter of 0.6 cm. The downstream section of thenozzle 203 of the nozzle 200 is smoothly faired away from the throat 202to a diameter of 1.3 cm at the nozzle exit 204 in a length of 10 cm. Thepressure in the downstream section 300 is maintained at 1.0 kPa by usingan external vacuum pump connected to conduit 301. The passage of the gasand particles through the nozzle 200 results in a supersonic stream 10of sodium chloride vapor 1.3 cm in diameter, with a core 11 of diameter0.6 cm where the particles are concentrated. The gas and particles inthe stream achieve velocities of 1100 m/s and 800 m/s, respectively. Ata distance of 2.0 cm from the nozzle exit, a circular, 4.0 cm diameterconical impactor plate 401 with included angle, A₂, of 160° is placedperpendicular to the stream. The plate forms the front face of theimpactor chamber 408 and comprises a circular aperture 402 0.8 cm indiameter at its center. The aperture 402 forms the entrance of acylindrical porous ceramic tube 405 0.8 cm id×1.2 cm od×10 cm long intowhich argon slowly bleeds from the impactor plenum 403 in order to exerta pressure within the impactor chamber 408 slightly greater than thatexerted by the impacting supersonic stream, 25 kPa. The temperature ofthe argon and of the impactor chamber are maintained at 1400K to preventsodium chloride condensation in the region where the argon and sodiumchloride in the supersonic stream contact.

The supersonic stream striking the virtual impactor causes the formationof a shock wave 12 which stands about 0.65 cm away from the plate 401 Atthe shock wave, the gas velocity changes to less than Mach no.=1 and thegas turns aside while the particles continue to move in their originaldirection at 800 m/s. As the sodium chloride flows away from theimpactor plate, it encounters the walls of the downstream section 300where it condenses separated from the particles. The temperature of thewalls of the downstream section 300 may be regulated so that thecondensation of the salt vapor forms liquid which can be channeled to acentral collection vessel.

Approximately 6.0 cm from the shock wave 12 and 5.4 cm downstream fromthe aperture 402, the particles are slowed by drag forces to theirgravitational settling velocity. At this location, the flow of argon ismoving in a direction away from the aperture and the particles areentrained in the flow inside the impactor chamber 408. The entrainedparticles are transported to the end of the porous tube 405 and thenceto a chamber 500 containing a heated ceramic filter 501 where they areseparated physically from the argon. The ceramic filter is used towithstand the temperature of the argon (1400K); the temperature ismaintained as insurance that any salt vapor which diffuses into theimpactor chamber remains as a vapor and does not condense on theparticles and contaminate them.

Less than 1% of the salt vapor in the supersonic stream enters theimpactor chamber 408 and this is separated from the particles by the hotfilter 501.

While the invention has been described with reference to a preferredembodiment, it will be appreciated by those of ordinary skill in the artthat changes may be made to the method and apparatus without departingfrom the spirit and scope of the invention as a whole.

I claim:
 1. A method for separating particles from a gas containing saidparticles, comprising the steps of:accelerating said gas containing saidparticles to a supersonic flow velocity in a supersonic nozzle; forminga shock wave in front of a virtual impactor by impacting said supersonicflow against said virtual impactor wherein said virtual impactorcomprises a plate having an aperture therein and an interior chamber;separating said particles from said gas behind said shock wave with saidvirtual impactor wherein said particles substantially follow theoriginal direction of flow of the accelerated gas said particles passingthrough said aperture of said impactor and the gas separated from saidparticles is caused to change direction from its original direction offlow and wherein the pressure in said interior is at least as great asthe pressure between said shock wave and said plate so that there is asmall positive outward flow of gas from said interior chamber throughsaid aperture; collecting said particles in an interior chamber cavitydownstream of said aperture; and recovering the gas separated from saidparticles.
 2. The method of claim 1 wherein said plate is flat.
 3. Themethod of claim 1 wherein said plate is convex.
 4. The method of claim 1wherein said plate is concave.
 5. The method of claim 1 wherein said gascontaining said particles is accelerated to said supersonic flowvelocity in a supersonic nozzle wherein the cross-sectional shape andarea of said aperture are approximately the cross-sectional shape andarea of the throat of said nozzle.
 6. The method of claim 1 wherein saidparticles are in the range of 0.1 to 10 micrometers in diameter.
 7. Themethod of claim 6 wherein said supersonic flow velocity has a speed inthe range of Mach 1 to Mach
 10. 8. An apparatus for separating particlesfrom a gas containing said particles said apparatus comprising:a housinghaving an inlet and outlet, said inlet receiving said gas containingsaid particles; supersonic nozzle means for accelerating gas containingsaid particles to a supersonic velocity and passing it into said inlet;a virtual impactor means including a plate having an aperture thereinsaid housing and an interior chamber housing, said interior chamberhousing is positioned adjacent said plate so that said aperture ispositioned above said interior chamber housing, wherein said supersonicflow strikes said virtual impactor means wherein said nozzle and saidimpactor cause the forming of a shock wave in front of said virtualimpactor means and wherein said particles flow through said aperture ofsaid impactor into said interior chamber and the gas separated from saidparticles is caused to change direction from the original direction offlow; means for introducing an inert gas into said interior chamberthereby maintaining the pressure in said interior chamber at least asgreat as the pressure behind said shock wave causing a small net flow ofgas from said interior chamber through said aperture; particlecollecting means for collecting said particles in said interior chamber;and recovery means for recovering the gas separated from said particleswherein the gas separated from said particles exits said housing throughsaid outlet.
 9. The apparatus of claim 8 wherein said plate is flat. 10.The apparatus of claim 8 wherein said plate is convex.
 11. The apparatusof claim 8 wherein said plate is concave.
 12. The apparatus of claim 8wherein a porous tube connects said aperture and said interior chamberand said inert gas is introduced through the walls of said porous tubeand said porous tube provides a path for introducing said particles fromsaid aperture into said interior chamber.
 13. The apparatus of claim 8wherein the cross sectional shape and area of said aperture isapproximately the cross sectional shape and area of said nozzle.
 14. Theapparatus of claim 8 further comprising:filter means within saidinterior chamber for collecting said particles.
 15. The apparatus ofclaim 14 further comprising heater means wherein said filter means isheated by said heater means to prevent condensation of condensible gaseson said collected particles.